Solid-state reaction mechanism of columbite-to-pyrochlore conversion at different temperature

Columbite-to-pyrochlore conversion is proposed to enhance the difference of physicochemical properties between niobium minerals and gangue minerals with the aim of improving the niobium minerals processing of Bayan Obo deposit. Columbite mixed NaF and CaCO3 could be converted into pyrochlore. Gibbs free energies of the conversions were calculated by two-parameter model. The reaction mechanism was clarified through X-ray diffraction and scanning electron microscopy equipped with an energy dispersion spectrometer. Oxygen benefited the conversion because FeO, by-product of the reaction is oxidized into Fe2O3, releasing a large amount of heat. Experimental results indicated that NaF reacted with CaCO3 before columbite conversion. Columbite-to-NaNbO3 conversion was dominant as sintering temperature was from 873 K to 973 K. Columbite-to-pyrochlore conversion dominated with the temperature increasing to 1073 K although the reaction occurred at 973 K. Therefore, Columbite-to-pyrochlore conversion preferred to occur when columbite mixtures were placed into the tubular furnace with a temperature of 1073 K directly.